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| author | Bryan O'Donoghue <bryan.odonoghue@linaro.org> | 2023-09-28 02:58:25 +0200 |
|---|---|---|
| committer | Hans Verkuil <hverkuil-cisco@xs4all.nl> | 2023-10-07 10:55:44 +0200 |
| commit | f910d3ba78a2677c23508f225eb047d89eb4b2b6 (patch) | |
| tree | 88327b3c5a09dcaabd908180c14f676f7a94887d /drivers/media/platform/qcom | |
| parent | media: qcom: camss: Propagate vfe_reset error up the callstack (diff) | |
| download | linux-f910d3ba78a2677c23508f225eb047d89eb4b2b6.tar.xz linux-f910d3ba78a2677c23508f225eb047d89eb4b2b6.zip | |
media: qcom: camss: Comment CSID dt_id field
Digging into the documentation we find that the DT_ID bitfield is used to
map the six bit DT to a two bit ID code. This value is concatenated to the
VC bitfield to create a CID value. DT_ID is the two least significant bits
of CID and VC the most significant bits.
Originally we set dt_id = vc * 4 in and then subsequently set dt_id = vc.
commit 3c4ed72a16bc ("media: camss: sm8250: Virtual channels for CSID")
silently fixed the multiplication by four which would give a better
value for the generated CID without mentioning what was being done or why.
Next up I haplessly changed the value back to "dt_id = vc * 4" since there
didn't appear to be any logic behind it.
Hans asked what the change was for and I honestly couldn't remember the
provenance of it, so I dug in.
Link: https://lore.kernel.org/linux-arm-msm/edd4bf9b-0e1b-883c-1a4d-50f4102c3924@xs4all.nl/
Add a comment so the next hapless programmer doesn't make this same
mistake.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Diffstat (limited to 'drivers/media/platform/qcom')
| -rw-r--r-- | drivers/media/platform/qcom/camss/camss-csid-gen2.c | 14 |
1 files changed, 13 insertions, 1 deletions
diff --git a/drivers/media/platform/qcom/camss/camss-csid-gen2.c b/drivers/media/platform/qcom/camss/camss-csid-gen2.c index 6ba2b1032644..05ff5fa8095a 100644 --- a/drivers/media/platform/qcom/camss/camss-csid-gen2.c +++ b/drivers/media/platform/qcom/camss/camss-csid-gen2.c @@ -352,7 +352,19 @@ static void __csid_configure_stream(struct csid_device *csid, u8 enable, u8 vc) phy_sel = csid->phy.csiphy_id; if (enable) { - u8 dt_id = vc; + /* + * DT_ID is a two bit bitfield that is concatenated with + * the four least significant bits of the five bit VC + * bitfield to generate an internal CID value. + * + * CSID_RDI_CFG0(vc) + * DT_ID : 28:27 + * VC : 26:22 + * DT : 21:16 + * + * CID : VC 3:0 << 2 | DT_ID 1:0 + */ + u8 dt_id = vc & 0x03; if (tg->enabled) { /* configure one DT, infinite frames */ |